Electronic circuit systems are typically made by defining complex integrated circuit structures on semiconductor chips, bonding to chips to circuit package substrates, and in turn bonding the packages to printed circuit boards. The most common bonding technique is wire bonding, in which an instrument (a thermode) thermo-compression bonds wire to a bonding pad of one element, such as a chip, and then pulls the wire and makes a bond on a bonding pad of a second element so as to form an arcuate self-supporting wire bridge the two bonding pads. Other soldering techniques make use of self-supporting leads known as beam leads that extend from the chip. Others use leads that are supported in cantilever fashion by a tape which are applied to chips by a technique known as tape automated (TAB) bonding. With many of these techniques, it is not necessary to apply force to effect a bond; all that is required is the application of heat to melt (reflow) solder between the leads and the bonding pads.
Direct heating of circuit assemblies to reflow solder has been applied, for example, by hot air convention and by an infrared source. One disadvantage of such application is that a relatively large area of the product to be made must be heated to a temperature above the melting point of the solder. Laser beam soldering has been proposed in which the heat is applied by a laser beam directed against leads overlying the solder pads. Differences in reflectivity resulting from variable surface properties may create problems, and also positioning of the beam may be difficult to control. Moreover, it would be desirable, rather than directing a laser at each individual lead, to use a laser to bond a number of leads simultaneously. As the density of circuits increases, along with the increased microminiaturization of integrated circuit chips, there will be an increased need for more expedient soldering processes. Further, in soldering a circuit package to printed circuit board, impingement of the laser beam on the circuit board is likely to damage the circuit board.
The U.S. Pat. No. 4,404,453 of Gotman describes a method for soldering a chip to an alumina ceramic substrate in which a laser beam is directed at the under surface of the substrate to heat it, the heat being transmitted by the substrate to the bonding pads so as to melt the solder between the chip leads and the bonding pads. This has the disadvantage of requiring heating of the substrate to a substantially higher temperature than the melting point of solder, which could damage the substrate, and it is not applicable to printed wiring boards which are typically made of organic materials such as an epoxy-filled mesh.